Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
This is why I have cats! /s

I liked the suggestion to just hide the remote, much easier than taking and hiding the Apple TV.
 
Hehe. I just thought of a ridiculously simple one. Get one of the power outlet timers. Plug the Apple TV into it. Make sure no-one in the household sees you doing this (hopefully the power outlet is sufficiently hard to get to) 🙂

Don't plug the TV into it (that would be the dead giveaway to investigate the power from a troubleshooting standpoint).
 
Instead of trying to lock him out of the Apple TV, would using the Screen Time feature on the Mac solve this? Setting up Downtime would also keep him from just playing that media on the Mac instead. If you have Family Sharing set up, you can manage his devices (and his account on a shared computer) from your own.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.